1. Field of the Invention
The present invention relates to a conductor for wiring and terminal connection, and in particular, relates to a conductor used for a flexible substrate, such as a flexible flat cable (FFC) and flexible printed-wiring board (FPC), used for electronic devices and a fabrication method of the conductor. Furthermore, the present invention relates to a flexible substrate using the conductor.
2. Description of Related Art
FIG. 2 is a schematic illustration showing a perspective view of an example of a joint between a connector and a flexible flat cable. As shown in FIG. 2, in a terminal connection at which a flexible flat cable (hereafter, referred to as FFC) 13 is connected to a connector 11, plating is applied to surface of a connector pin (metal terminal) 12 of the connector (connector member) 11 and a conductor 14 of the FFC 13. Conventionally, Sn (tin), Ag (silver), Au (gold) or Ni (nickel) plating is applied to a surface of wiring material, specifically copper and copper alloy, in order to prevent the wiring material from oxidizing. Especially, the cost of raw material for Sn is low, and Sn is soft. Therefore, Sn is easily deformed by pressure at a contact point, increasing a contact area, and thereby making it possible to keep the contact resistance low. For this reason, wiring material with its surface plated with Sn is widely used.
An Sn—Pb alloy having a good resistance to whiskers has been conventionally used as an Sn plating alloy. However, recently, from the viewpoint of environmental considerations, the use of Pb-free material (lead-free material) and non-halogen material are desired. Therefore, it is required that material used as wiring material be Pb-free and non-halogen, and a variety of research and development have been carried out. (For example, JP-A-2006-111898, JP-A-2005-216749, JP-A-2005-206869, JP-A-2006-45665, Reference for JEITA's lead-free achievement urgent proposal meeting (2005.2.17), and JEITA's 2005 research report about practical application of lead-free soldering (2005.6))
However, there is a problem with the development of Pb-free technology of the Sn plating, specifically in the Sn or Sn-group alloy plating. FIG. 3 is a schematic illustration showing an enlarged perspective view for explaining appearance (growth) of whiskers on the joint between the connector and the FFC. As shown in FIG. 3, whiskers 21 that are acicular crystals of Sn appear (grow) from the plating, thereby causing short-circuits between adjacent conductors.
It is considered possible to reduce appearance (growth) of whiskers by executing the reflow process of electroplated Sn so as to reduce a residual stress in the Sn plating which is considered one of the causes for the appearance (growth) of whiskers. However, a detailed mechanism to inhibit whiskers has not yet been clarified. Furthermore, in the cases in which external stress is newly applied to the contact point in the joint of the connector or the like after the reflow process, the application of the reflow process before the joint will not be helpful to inhibit the appearance of whiskers. On the other hand, although it is possible to suppress whiskers by applying Bi (bismuth) or Ag alloy electrolytic plating or electroless plating, there is a report indicating that the application of the reflow process may cause whiskers to appear more frequently than in the case of the pure Sn plating. Besides, it is assumed that the reflow process is a necessary process for electronic components in order to mount components, therefore, creating a problem with the use of this alloy (Bi or Ag alloy) plating.
Currently, an effective method is disclosed in which thin Sn plating with the thickness of 1 μm or less is applied. However, in this method, there is a problem in that contact resistance increases more than in the conventional technological methods, specifically when the plating is left under high temperatures.